Fix indentation and use gradle docker image

The .yaml cannot be parsed if the indentation is wrong. Gitlb CI Lint
does not find that and simply complains about the first line.

A gradle docker image is used now. Hopefully that works for us, there is
no official gradle docker image. Alternatively, we could use a java
image, but we would probably need to install gradle on that, so I hope
taking this image makes things easier.

.gitlab-ci.yml

-# This template uses the java:8 docker image because there isn't any
-# official Gradle image at this moment
-#
-# This is the Gradle build system for JVM applications
-# https://gradle.org/
-# https://github.com/gradle/gradle
-image: java:8
+image: frekele/gradle:latest
 
 # Make the gradle wrapper executable. This essentially downloads a copy of
 # Gradle to build the project with.